the two terminals on top is your LINE input. the two bottom ones are your LOAD terminals. the bar in the middle is your neutral wire connections. let me caution you that what you are installing is called a sub-panel, the wiring rules are different than regular romex. your feeder size is #10 gauge. you cannot run romex inside conduit of any type except for mechanical protection where the wire enters or exits the ground trench. you need four wires run out to your pullout. two hots, a neutral and an equipment ground [case ground] the neutral bar, the thing in the center cannot be bonded to the case. #10-3 type UF with ground will satisfy the requirement. the wire needs to be buried 24 inches down in rock free sand and covered with sand, then your topsoil goes over that.